Capacitive humidity sensor

ABSTRACT

Capacitive humidity sensor comprising a substrate, a moisture-impermeable conducting bottom layer as first capacitor plate, a dielectric layer, and a moisture-permeable conducting top layer as second capacitor plate, and connecting wires associated with the first and second capacitor plates, whereby the change in the dielectric constant, and therefore in the capacitance value, due to absorption of water molecules is measured. The substrate is a flexible copper laminate in which at least the conducting bottom layer is etched, and the conducting top layer comprises a solid printed layer having a conducting grid pattern, which is printed with conducting ink.

BACKGROUND OF THE INVENTION

The invention relates to a capacitive humidity sensor comprising asubstrate being a flexible copper laminate, a moisture-impermeableconducting bottom layer etched in said copper laminate as firstcapacitor plate, a dielectric layer of which the dielectric constant isa function of moisture absorption, and a moisture-permeable conductingtop layer as second capacitor plate, said bottom layer and a contactlayer making contact with the moisture-permeable top layer and beingdeposited adjacent each other and isolated from each other on thesubstrate, the top layer extending both over the dielectric layer andover the contact layer. Such a humidity sensor is disclosed in Europeanpatent application 0094266.

In such a sensor, which in fact consists of an electrical capacitor, thehumidity is determined by measuring the change in dielectric constant ofthe dielectric medium, and therefore the change in capacitance value,due to the absorption of water molecules by the dielectric material. If,for example, the material of a dry dielectric layer has a dielectricconstant of 3.5 and water approximately has one of 80, it will be clearthat the change in capacitance value due to moisture absorption may beappreciable. In order that moisture or water can be absorbed by thedielectric layer, the top layer has to be moisture-permeable.

This known sensor has a substrate on which a copper layer as firstcapacitor plate is deposited. A moisture-impermeable insulating layer isprovided on top of the first capacitor plate. Said insulating layerserves as a barrier layer and prevents resistance loss across thedielectric medium at high relative humidity, or even short circuitingbetween the plates regardless of the structure of the dielectric medium.The dielectric layer mentioned is sufficiently porous to absorbmoisture.

Now in practice it appears that the use of such a sensor is strictlydependent on well-defined precalibration techniques under dry and moistatmospheric conditions before using it for the consumer market and forindustrial application.

The object of the invention is to eliminate the above mentioned problemand to provide a cheap sensor which is most robust and which can beeasily calibrated after manufacture.

SUMMARY OF THE INVENTION

According to the invention this is achieved in a capacitive humiditysensor of the type mentioned above in that the top layer comprises asolid printed grid pattern of conducting ink, and a row of smallconducting ink areas at at least one side of the grid pattern.

In an advantageous embodiment of the above sensor at least between thebottom layer and the dielectric layer a moisture-impermeable insulatinglayer or barrier layer is provided, whereby in one case the row of smallink areas is provided above the insulating layer and in another case tworows of small ink areas on either side are provided of which one isabove the insulating layer and the other is above the dielectric layer.

In this embodiment according to the invention, an exceptionally cheapsensor for the consumer market is obtained which is also suitable forindustrial applications. This sensor according to the invention can becalibrated simply, both under dry and moist atmospheric conditions. Forcalibration in the case of a dry reference environment, several of thesmall areas of the row above the barrier layer are connected to the maingrid by means of small pads of conducting ink in order to obtain therequired fixed (dry) capacitance value. For further calibration in thecase of a humid reference environment, small areas of the row above thedielectric or active layer are connected to the main grid in order toadjust the required delta capacitance value. For this calibration, it isassumed that it is easier to add than to subtract a capacitance value.Such a subtraction may, however, also be achieved by laser trimming.After trimming, the conducting ink tracks can be hardened. Theadvantages of this "printing" method are that the exact printinggeometry can be defined and there is no dependence on fortuitousphenomena. Trimming can be carried out both for the fixed and the deltavalues. If process faults are made during printing, these can be"erased", the more expensive part of the substrate clad with the barrierlayer being left intact.

From French patent application 2327536 a capacitive humidity sensor isknown consisting of a simple structure of a copper plate as firstcapacitor plate, dielectric paper as dielectric medium and an uppercapacitor plate which consists of a copper wire grid. Said wire grid isfastened with the aid of plastic screws to the first capacitor plate.This humidity sensor is intended to detect the presence of oil in gas.

From German patent application 3203990 a method is known for adjustingthe capacity of a capacitive humidity sensor. This adjustment isperformed by using a laser beam to remove or trim the conducting uppercapacitor plate consisting of a gold film. This is performed at hightemperature in order to cause the remaining moisture, still present inthe dielectric medium, to be negligibly small.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail on the basis of anexemplary embodiment with reference to the drawing, in which:

FIG. 1 shows a perspective view of the layered structure of theexemplary embodiment of a sensor according to the invention; and

FIG. 2 shows a plan view of the top layer, which acts as secondcapacitor plate, of the exemplary embodiment of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As stated above, the object of the invention is to provide a cheapsensor for consumer applications.

According to the invention, the glass substrate material, which is perse an excellent material, is now replaced in practice by copper laminatewhich is frequently used, for example, in printed circuit boards (PCBs).In a further embodiment, said copper laminate may be constructed withpolysiloxane-glass textile or fabric. The moisture-impermeable bottomlayer can be etched on the copper surface of such a laminate as firstcapacitor plate, if necessary with a second thinner contact plate nextto it which acts as contact plate for the moisture-permeable top layer.Connecting wires can be attached to said bottom layer and contact layerin a later phase of the manufacture.

The use of such a thin flexible PCB laminate has the advantage that usecan be made of PCB manufacturing techniques, the finished result alsoyielding a more robust sensor than the one manufactured from thin-filmglass substrates. It is then possible to start with larger substrates,for example, resulting in a quantity of 50×50=2500 sensors. Undercertain conditions, in which an insulating polymer is deposited asinsulating layer on the moisture-impermeable bottom layer, it ispossible in practice to process two substrates simultaneously back toback, as a result of which a quantity of 5000 sensors can be produced inone processing phase.

FIG. 1 shows diagrammatically the structure of a sensor according to theinvention.

1 indicates the substrate consisting of a thin PCB copper laminate inwhich two copper layers 2 and 3 are etched. The layer 2 indicated at theleft-hand side is the moisture-impermeable layer which acts as firstcapacitor plate and the layer 3 is the contact plate which makes contactwith the moisture-permeable top layer 4, which acts as second capacitorplate, in a later phase of the manufacture. When the substrate is cut upin a final phase and the individual sensors are separated, theconnecting wires 5 are attached to the layers 2 and 3.

6 indicates a polymer coating which functions as insulating layer andwhich, although extremely thin, protects the surface of the activecopper capacitor plate 2 against attack and contamination. Since thelayer 6 is itself moisture-impermeable, any short circuiting between thecapacitor plates mentioned is consequently prevented, regardless of thestructure of the dielectric material, and direct current, which wouldotherwise produce polarization drift as a consequence of electrolysis,is prevented from flowing. The layer will prevent breakdown occurring asa consequence of the occurrence of so-called "pinholing" in the activedielectric 7. The polymer 6 does not extend over the thinner coppercontact layer 3. In the latter case, an inert, non-reactive pinhole-freebarrier polymer is preferably used.

The dielectric layer 7 is composed of an active polymer which can bespun, can be deposited by means of screen printing or in another manner,so that an extremely thin layer is produced which nevertheless containsa sufficient quantity of pores required for the operation of thehumidity sensor. In this manner, a dielectric "sponge" is obtained whichensures a rapid change in the capacitance value under the influence ofmoisture. A large "inhalation" and "exhalation" surface is consequentlyachieved. The figure indicates that the layer 7 does not extend farenough to lie on top of the contact layer 3. But other methods ofmanufacture are conceivable.

4 indicates the moisture-permeable top layer as second capacitor platewhich extends over the whole. The part of said top layer extending overthe dielectric layer 7 has the form of a grid printed on the dielectriclayer with conducting ink.

The plan view shown in FIG. 2 shows the top plate 4 mostly as arectangular grid pattern, although other patterns are also possible. Thegrid openings ensure moisture transmission. The geometry of the grid 10is important. A maximum (conducting) ink surface area will produce amaximum capacitance value, but then the time necessary for absorptionand desorption will be large, unless the moisture-transmitting surfacearea is sufficiently large. A good result is produced with a relativelylarge sensor surface area, such as, for example, 10 mm², in which casethe response time is adversely affected to a certain extent.

11 indicates the part of the top plate which is in solid printed form,i.e. does not have any openings, and which is intended to make contactwith the contact layer 3 on the substrate 1.

12 indicates a row of small conducting printed areas in an insulatedarrangement. In constructing the capacitor, it is intended that said rowof small areas 12 is situated only above the barrier polymer and notabove the active polymer. The capacitances thereby formed have a minimumor zero value.

In manufacture, the design is such that the sensor lies somewhat belowits normal tolerance of capacitance values, both under dry and moistconditions (delta=0% to 100% relative humidity range). For automaticcalibration, several of the small areas 12 are connected in the case ofa dry reference environment to the main grid by means of small pads ofconducting ink in order to obtain the required fixed (dry) capacitancevalue.

13 indicates another row of small conducting printed areas in aninsulated arrangement. In the construction of the capacitor, however,these are situated above the active polymer. These small areas can alsobe connected to the main grid in order to adjust the required deltacapacitance value in a humid or moist reference environment.

In the above, it has been assumed that it is easier to add than tosubtract a capacitance value. Such a subtraction may, however, also beachieved by laser trimming.

After trimming, the conducting ink tracks can be hardened. Theadvantages of this "printing" method are that the exact printinggeometry can be defined and there is no dependence on fortuitousphenomena. Trimming can be carried out both for the fixed and the deltavalues. If process faults are made during printing, these can be"erased", the more expensive part of the substrate clad with the barrierpolymer being left intact.

The small trimming areas are shown as small squares. It will be clearthat they can also be provided in a smaller number and, for example, ina binary surface area progression.

During manufacture, sensors can be manufactured in accordance with therequired application having an improved response speed by making onedimension greater, as a result of which a longer sensor is obtained.Since the contact surface occupies an appreciable part of the totalsurface, a more slender design will make it possible for the contactsurface area to decrease in favor of a larger active surface area.

Since the barrier polymer is expensive, the copper laminate ispreferably clad only on one side. This can best be achieved byprocessing the substrate plates two at a time, back to back. To do this,it is necessary to mask the contact layer or even to grind off thebarrier layer obtained. Since the laminates can be obtained processed onboth sides, it is possible to provide both contact layers on the otherside, unclad by the barrier layer and consequently readily solderableand to connect them by means of through-hole plating or by means ofspecially designed "knife-edge" contact terminals.

All the inactive surfaces which are not covered by the barrier polymercan be covered with solder resist in the normal manner of protecting PCBlaminates.

The cheap humidity sensor specified above, which can be manufactured ina reproducible manner, may be used for many types of applications, suchas for air treatment systems, in the motor vehicle industry for windowdemisting systems, brake (shoe) systems and for internal combustionengines in which the combustion process is strongly dependent on therelative humidity.

I claim:
 1. Capacitive humidity sensor comprising a substrate being aflexible copper laminate, a moisture-impermeable conducting bottom layeretched in said copper laminate as first capacitor plate, a dielectriclayer of which the dielectric constant is a function of moistureabsorption, and a moisture-permeable conducting top layer as secondcapacitor plate, said bottom layer and a contact layer making contactwith the moisture-permeable top layer being deposited adjacent eachother and isolated from each other on the substrate, the top layerextending both over the dielectric layer and over the contact layer,wherein the top layer comprises a solid printed grid pattern ofconducting ink, and a row of small conducting ink areas at at least oneside of the grid pattern.
 2. Capacitive humidity sensor according toclaim 1, wherein at least between the bottom layer and the dielectriclayer a moisture-impermeable insulating layer is provided, and the rowof small ink areas of the top layer is situated above the insulatinglayer and not above the dielectric layer, so that for precalibration ofthe capacitance value of the sensor in a dry atmosphere small ink areascan be connected to the grid pattern.
 3. Capacitive humidity sensoraccording to claim 1, wherein at least between the bottom layer and thedielectric layer a moisture-impermeable insulating layer is provided,and the row of small ink areas of the top layer is situated above thedielectric layer and not above the insulating layer, so that forprecalibration of the capacitance value of the sensor in a moistatmosphere small ink areas can be connected to the grid pattern. 4.Capacitive humidity sensor comprising a substrate, amoisture-impermeable conducting bottom layer as first capacitor plate, adielectric layer, and a moisture-permeable conducting top layer assecond capacitor plate, and connecting wires associated with the firstand second capacitor plates, the change in the dielectric constant, andtherefore in the capacitance value, due to absorption of water moleculesbeing measured, the substrate being a flexible copper laminate in whichat least the conducting bottom layer mentioned is etched, the conductingtop layer comprising a solid printed layer having a conducting gridpattern with sides thereof, the bottom layer and a contact layer makingcontact with the moisture-permeable top layer being deposited parallelto each other on the substrate, a moisture-impermeable insulating layerbeing present at least between the bottom layer and the dielectriclayer, wherein the top layer has a row of small ink areas at at leastone of the sides of the grid pattern.
 5. Capacitive humidity sensoraccording to claim 4, wherein the row of small ink areas in the sensoris situated such that the dielectric layer is not situated under the rowof small ink areas but the insulating layer is, so that, duringprecalibration of the capacitance value in a dry atmosphere, small inkareas can be connected, if necessary, to the grid pattern.
 6. Capacitivehumidity sensor according to claim 4, wherein the row of small ink areasin the sensor is situated such that the dielectric layer is situatedunder the row of small ink areas and the insulating layer is not, sothat, during precalibration of the capacitance value in a moistatmosphere, small ink areas can be connected, if necessary, to the gridpattern.